1. Field of the Invention
This invention relates generally to the fabrication of tin oxide semiconductor heterojunction devices. More particularly, the present invention relates to a method for consistently preparing high efficiency tin oxide semiconductor heterojunction solar cells by the hydrolysis on the surface of an N-type semiconductor of a vapor deposited solution of a tin compound.
2. Prior Art
There has been considerable interest in recent years directed toward the development of semiconductive heterojunction devices comprising an electrically conductive transparent window material deposited on the semiconductor substrate. Such interest is based on the potential that such devices can be fabricated at lower cost than homojunction devices based on the conventional diffusion of N and/or P materials into the semiconductor substrate. Additionally, heterojunction conducting oxide devices offer the added advantage that the transparent window permits the transmission of solar radiation directly to the active substrate with little or no attenuation. One such type of heterojunction solar cell proposed is a tin oxide semiconductor heterostructure solar cell, especially a tin oxide silicon heterostructure solar cell.
Transparent conductive tin oxide coatings on semiconductive substrates such as silicon have been formed by electron beam techniques (see, for example, Journal of Electronic Materials, Vol. 6, No. 2, pp. 107-123 (1977); and Report NSF/RANN/SE/AER67-04168/PR/77/1,2 (1977)) and by what is broadly characterized herein as chemical vapor deposition techniques (see, for example, Japan Journal of Applied Physics, 6, p. 905 (1967); U.S. Pat. Nos. 3,210,214; 3,679,949 and 3,952,323).
In general, none of the foregoing techniques have consistently resulted in heterostructure solar cells having high sunlight conversion efficiencies; however, from an economic and commercial point of view, a process for the fabrication of tin oxide-semiconductor heterostructure solar cells via vapor deposition techniques would be most desirable.
As will be appreciated, in the vapor deposition of transparent conductive tin oxide coatings, volatile compounds of tin are employed which are either hydrolyzed, oxidized, reduced or decomposed on a heated substrate. For a recent review of preparative techniques for depositing tin oxide films, see Journal of the Electrochemical Society, 123, No. 7, 199C-205C (1976). As is indicated in the just-mentioned reference, the most common and promising method of depositing tin oxide films depends upon the hydrolysis of inorganic tin halide compounds on the surface of a heated substrate. Typically a compound such as stannic chloride is dissolved in a solvent such as an alcohol, acetic acid or a mixture of the two, and the solution is sprayed, in air, onto the surface of the preheated substrate where the stannic chloride is hydrolyzed to form tin oxide. The solvent is used to modify the otherwise rapid conversion of the tin halide to tin oxide (see, for example, Australian Journal of Applied Science, 5, 10-17 (1954)).
One difficulty with tin oxide coatings deposited by such prior art techniques is the resultant low conductivity of the coatings; and hence the need to admix antimony trichloride or other dopants with the tin compound being vaporized, since the dopant apparently increases the electrical conductivity of the tin oxide coatings formed therewith. A similar requirement is disclosed in U.S. Pat. No. 3,679,949 wherein a conductive transparent coating of tin oxide is deposited on a semiconductive substrate by oxidizing a halogenated organic tin compound, preferably in admixture with SbCl.sub.3, on the heated semiconductor substrate. Nonetheless, tin oxide-silicon solar cells prepared by such techniques exhibit relatively low sunlight engineering efficiencies.
Thus, although it is well known that tin oxide semiconductor heterojunction devices can be formed by vapor deposition of tin oxide on the semiconductive substrate, there remains a need for a simple method of fabricating devices that will have high sunlight conversion efficiencies.